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16.2. Swap Area

The pages swapped out from memory are stored
in a swap area, which may be implemented either
as a disk partition of its own or as a file included in a larger
partition. Several different swap areas may be defined, up to a
maximum number specified by the
MAX_SWAPFILES macro (usually set to 8).

Having multiple
swap areas allows a system administrator to spread a lot of swap
space among several disks so that the hardware can act on them
concurrently; it also lets swap space be increased at runtime without
rebooting the system.

Each swap area consists of a sequence of page
slots
, that is, of 4096-byte blocks used to
contain a swapped-out page. The first page slot of a swap area is
used to persistently store some information about the swap area; its
format is described by the
swap_header union composed of two structures,
info and
magic. The magic
structure provides a string that marks part of the disk unambiguously
as a swap area; it consists of just one field,
magic.magic, containing a 10-character
"magic" string. The magic structure
essentially allows the kernel to unambiguously identify a file or a
partition as a swap area; the text of the string depends on the
swapping algorithm version. The field is always located at the end of
the first page slot.

The info structure includes the following
fields:

info.bootbits

Not used by the swapping algorithm; this field corresponds to the first 1024 bytes of the swap area, which may store partition data, disk labels, and so ...

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